Cascade polyene cyclization reactions are highly efficient and elegant bioinspired transformations that involve simultaneous multiple bond constructions to rapidly generate complex polycyclic molecules. This review summarizes the most prominent work on a variety of cationic and radical cascade cyclizations and their applications in natural product synthesis published between 2014 and 2018.1 Introduction2 Cationic Polyene Cyclizations2.1 Lewis Acid Mediated Polyene Cyclizations2.2 Brønsted Acid Mediated Polyene Cyclizations2.3 Halogen Electrophile Initiated Polyene Cyclizations2.4 Sulfur Electrophile Initiated Polyene Cyclizations2.5 Transition-Metal-Mediated Cationic Polyene Cyclizations3 Radical Polyene Cyclizations3.1 Transition-Metal-Mediated Radical Polyene Cyclizations3.2 Photocatalyst-Mediated Polyene Cyclizations4 Origin of Stereocontrol in Polyene Cyclizations5 Conclusion

The synthesis and structural characterization of trans-[IrCl(NO)(PtBu 2 H) 2 ]BF 4 (1) is reported. The title compound was prepared in good yield in the reaction of in situ-prepared trans-[IrCl(coe)(PtBu 2 H) 2 ] (coe = cis-cyclooctene) with an appropriate Recently we reported the synthesis and molecular structure in the crystal of the nitrosyl compound trans-[RhCl(NO) (PtBu 2 H) 2 ]BF 4 . [1] The latter was obtained in good yield by a kind of "bridge-splitting" reaction of [{Rh(μ-Cl)(PtBu 2 H) 2 } 2 ] with a corresponding amount of nitrosonium tetrafluoridoborate in dichloromethane at room temperature. Complexes of the type trans-[IrX(NO)(PPh 3 ) 2 ] + (X = Cl, OH, OR) were described in the literature long ago, [2] but no crystal structure reports for these species have been published till now. Moreover the synthesis of 15 N enriched compounds with the formula trans-[RhCl( 15 NO)(PiPr 3 ) 2 ]X (X = ClO 4 or BF 4 ) were described, [3] however, even in the case of rhodium no reports on crystal structures have hitherto been reported for such species. Because no hints at the existence of complexes [{Ir(μ-Cl)(PR 3 ) 2 } 2 ] serving as possible starting compounds in the above mentioned "bridge-splitting" reaction exist in the literature, we searched for other suitable precursor compounds to prepare complexes of the general formula [IrX(NO)(PR 3 ) 2 ] + .In this light we proofed the usefulness of complex trans-[IrCl(coe)(PtBu 2 H) 2 ] [4] (coe = cis-cycclooctene) as a possible precursor for the preparation of the envisaged nitrosyl compounds. Indeed, during this study we could obtain the new compound trans-[IrCl(NO)(PtBu 2 H) 2 ]BF 4 (1) in good yield and were able to confirm its molecular structure by a singlecrystal X-ray diffraction study.

Amorfrutin C, a C-5 prenyl amorfrutin, its allyl analog and an amorfrutin C-5 aldehyde have been synthesized using a biomimetic strategy from non-aromatic precursors. In this approach, a dioxinone derived β,δ-diketo ester underwent a decarboxylative Pd(0) catalyzed prenyl migration to give a β,δdiketo dioxinone, which readily aromatized giving the amor-frutin core. The introduction of prenyl and allyl moieties at the C-5 position of the scaffold was accomplished using a Claisen rearrangement. Alternatively, iodination, lithium-iodine exchange and trapping with DMF gave the amorfrutin aldehyde and an amorfrutin alcohol when excess n-BuLi was used.

The hongoquercins are tetracyclic meroterpenoid natural products with the trans–transoid decalin-dihydrobenzopyran ring system, which display a range of different bioactivities. In this study, the syntheses of a range of hongoquercins using gold-catalyzed enyne cyclization reactions and further derivatization are described. The parent enyne resorcylate precursors were synthesized biomimetically from the corresponding dioxinone keto ester via regioselective acylation, Tsuji-Trost allylic decarboxylative rearrangement, and aromatization. The dioxinone keto ester 12 was prepared in 6 steps from geraniol using allylic functionalization and alkyne synthesis.

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